Noise suppression is conventionally performed, for example, in a vehicle mounted car navigation system, a hands-free phone, or a telephone conference system, to suppress noise contained in a speech signal that has mixed-in noise other than a target voice (for example a person's speech). A technique employing a microphone array including plural microphones is known as such noise suppression technology.
In such conventional noise suppression technology using a microphone array, a method has been disclosed in which a phase difference computed from respective input signals to each of the microphones in the microphone array is employed to derive a value representing the likelihood of a sound source being in a specific direction. In this method, based on the derived value, sound signals from sound sources other than the sound source in the specific direction are suppressed. A method has also been described that utilizes an amplitude ratio between input signals of each of the microphones to suppress sound other than from a target direction.
For example, a technique has been proposed that respectively divides waveforms acquired at two points into plural frequency bands, derives time differences and amplitude ratios for each band, and eliminates waveforms that do not match an arbitrarily determined time difference and amplitude ratio. In such a technique, after waveform processing and laying out each of the bands alongside each other, it is possible to selectively extract only the sound of a source at an arbitrary position (direction) by adding together the outputs of each of the bands. Moreover, in this technique, when selectively extracting sound from a sound source that has a difference in distance from two microphones, the phase difference or amplitude ratio are aligned with each other by performing signal delay or amplitude amplification, and then waveforms whose phase difference or amplitude ratio do not match are removed.
There has also been a proposal for a technique in which phase differences are detected between microphones by employing a target sound source direction estimated from the sound received from two or more microphones, and then using the detected phase differences to update a central phase difference value. In such a technique, a noise suppression filter generated using the updated central value is employed to suppress noise received by the microphones, and then sound is output.
There has also been a proposal for a technique in which audible signals received from two sensors placed in various different places are converted, spectral signals arise, a spectral signal is delayed, and many intermediate signals are supplied. Each of the intermediate signals corresponds to different spatial positions with respect to the two sensors, and the locations of noise sources and a desired emitting source, together with the spectral content of the desired signal, are determined from the intermediate signals corresponding to the location of the noise source.